In general, in driving, things happen too fast for any control assist system to allow hands off driving.
EDIT: Any existing control assist system
And I don't think we are that close, since "self driving cars" still seem to be limited to "driving at 25 MPH around Mountain View and Sunnyvale where the curbs and streets have been pre-mapped to within inches".
I see you've never skippered a boat.
I assume, just like with cars, things can happen quickly with a boat.
The autopilot for a aircraft does whatever it is supposed to do without human supervision. If you turn on keep heading, it will keep the heading (within physical limitations). I guess it is similar for a boat.
Lane assist, however, requires constant human supervision because it is just an assistant. I think this is the important difference that needs to be highlighted here.